1. Technical Field of the Invention
The present invention relates to a method and an apparatus for inspecting defects on a surface or section of an object to be inspected, such as a silicon wafer.
2. Related Art
A microscopic inspection for possible detects on a surface or section of an object to be inspected, such as a silicon wafer, has already been carried out. Because the field of view of a normally used microscope is approximately a several-millimeters square, inspecting a whole area of a surface to be inspected, for example, of a silicon wafer requires that the field of view of the microscope be moved several tens of times or several hundreds of times repeatedly. Thus it takes a lot of time for an operator to perform the inspection.
The technique for overcoming such a situation has been proposed by Japanese patent laid-open (unexamined) publications 11(1999)-354599 and 2001-183301.
In the former publication, a differential interference microscope with an objective lens is used. At first, with the magnification of the objective lens adjusted to a lower value, a one-dimensional CCD camera is operated to image a surface of a silicon wafer through a wider inspection field of view. Coordinates of luminescent spots detected by an optical system are recorded for inspecting the whole surface of the silicon wafer. Secondary, the magnification of the objective lens is changed to a higher value and the positions of the luminescent spots that have been detected by the optical system are imaged. The resultant image is used for a defect inspection. This inspection technique makes it possible to omit the locations having no defects from being imaged and inspected, whereby the inspection time can be shortened.
The inspection technique disclosed by the latter publication is realized by using two optical systems. Concretely, the two optical systems for both of a bright view and a dark view are placed for imaging, in which on the basis of the positions of defects detected in the dark field, a bright-field image at the positions is displayed for the inspection.
However, the inspection techniques disclosed by the above two publications have still difficult situations as below. In the case of the technique according to the publication 11(1999)-354599, the one-dimensional CCD camera installed in the differential interference microscope is used to image a surface of the silicon wafer or others. This results in that one field of view is not less than a square of a few millimeters, even if the magnification of the objective lens is adjusted to a lower value to widen the filed of view for imaging. Hence, this inspection technique has still suffered from the problem that it takes a lot of time to scan the whole area of a surface to be inspected of an object such as a silicon wafer.
In addition, when blots or others on an object, which are produced during, for example, an etching operation, are observed at a higher magnification, the blots or others are imaged as being luminescent spots. This makes it difficult to distinguish the blots or others from the defects. As a result, during a final inspection step carried out at higher magnifications, there often occurs the necessity of re-imaging the whole area of the surface of the object, which will reduce the efficiency of inspection.
On the other hand, the technique according to the publication 2001-183301 has been realized by adopting two different optical systems, where the inspecting magnifications assigned to the optical systems are approximately equal to each other, thus increasing detection sensitivity. However, since the inspection time is about two times longer than the conventional detection systems, the efficiency of inspection is suppressed to lower values, which makes it difficult to apply this detection technique to practical use.